The fabrication of semiconductor devices involves processing a substrate, such as a semiconductor wafer, to form various integrated circuit features on the substrate. One of the fundamental steps of any semiconductor fabrication process is photolithography, which is a series of steps for building layers of a three-dimensional circuit structure on the substrate. In each photolithography step, a light sensitive photoresist material is applied to the substrate through a mask, exposed to a light source, developed and then etched to form a portion of the three-dimensional structure.
The patterns formed during lithography directly affect the viability and fidelity of the intended integrated circuit features that are ultimately formed. While particles and defects are undesirable at any stage of the fabrication process, mask defects are particularly problematic since they will affect many different devices. Consequently, any defects formed as a result of lithography, such as the transfer of defects that are present on the mask, are problematic for the integrated circuit manufacturing process. Thus, inspection of masks for defects is an important part of quality control for the manufacturing process.
Fabrication processes typically include an inspection tool for inspecting masks for defects in order to predict whether the projected pattern image will faithfully reproduce the intended design of the device. In general, an inspection tool images a test image and a reference image on the mask and detects defects from processing the images. In many cases, an optical inspection apparatus does a good job of resolving the desired patterns on the mask, and sophisticated detection methods may be employed to separate the desired features of the image from defects that are undesirable.
However, for some lithographic methods, the masks are formed with high-resolution patterns that are not resolved well or perhaps not at all by the optical inspection apparatus, and therefore defect detection can be difficult in these cases. For example, an optical inspector having a numerical aperture (“NA”) of 0.75 and an inspection wavelength of 200 nm has a “nominal resolution” of 130 nm for 4× features on the mask, which is equivalent to 35 nm resolution on a wafer. Thus, the inspector has a hard optical cutoff for features having a half-pitch of 19 nm on the wafer. There is current interest in extreme ultraviolet (“EUV”) lithography to produce, for example, half-pitch lines at 16 nm, which would be completely invisible to the inspector described above. Further, efforts to produce even finer half-pitch lines with nano-imprint lithograph (“NIL”) are underway, for example, down to 6 nm half pitch, which would also be invisible to the inspector mentioned above.
Using conventional methods, at least two images are compared, which requires significant computing power and memory to choose or form the reference, to do subtractions, and to make the images as similar as possible for processing. Also, subtraction of images may result in the noise coming from both images combining to form a total noise component which is larger for the difference than for either of the contributors. Thus, it would be desirable to have alternative methods for inspecting, particularly for high resolution masks.